We told you last month about the seven-minute workout. You're wasting time. Maybe you need a faster way to get fit. Just four minutes a day. Abc's claire shipman has more on what is certainly the quick et workout ever. Reporter: Forget posting about long workouts. That's so yesterday. Now, the shorter, the hotter. Just last month, I tried this seven-minute workout. And now, four minutes. Okay. Here we go. Here's the rub. 90% of your heart capacity for all four minutes. My trainer, aaron sterling, on-hand to help me through another potentially humiliating fitness challenge. Scientists in norway say four minutes is all you need to get in shape. One worked out four minutes, three times a week, at 90% heart capacity. A second group did the same thing, but did four four-minute intervals. Group one, the first group, had the same improvement as the second group, in endurance and heart health. There's no resistance training. You do need resistance training, to increase your resting metabolism for bone density. And posture. Reporter: My posture was lagging by the end. And the study's author says it doesn't promote as much weight loss as a longer workout might. But sterling says the efficiency and the results of high-intensity exercise, gives it the allure to get people moving. I think it's healthy that people are starting to see, you don't need to work out for as long as you might think you need to get the benefits. Reporter: Four minutes definitely has the advantage of seeming doable. Is a three-minute version far behind? I'm done. Yes. For "good morning america," claire shipman, abc news, washington.

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